Bicomponent fibers are composed of two polymer compositions which are concentrated in separate areas of the filaments. In some prior art types, each polymer composition is continuous along the entire fiber length and the two components are permanently joined at an interface so as to form a side-by-side arrangement. Polymer compositions forming the components are selected on the basis of their ability to shrink or swell to different extents in response to hot-wet conditions. As a result, when fibers formed from the selected combination of polymer components are properly treated, differential shrinkage of the components will occur and a spiral or helical crimp will form. The extent of crimp development will depend upon the shrinkage differential between polymer components employed, distribution of components in the fiber and the presence of translational restraints which may inhibit crimp development.
Two different types of bicomponent acrylic fiber are usually produced. One contains different amounts of water-ionizable groups in the two polymer components and, as a consequence, the more hydrophilic side of the resulting fiber swells more in water. Crimp develops when the fiber is dried after hot-wet treatment. The spiral crimp thus developed is water-reversible, it decreases upon wetting while exhibiting squirming and reforms upon drying In the second bicomponent type, the two polymer components contain different amounts of nonionic comonomers. Crimp develops in such fiber when the oriented fiber is treated under conditions where adequate differential shrinkage develops between the two polymer components. Once formed, this helical crimp is permanent and unaffected by subsequent wetting and drying.
In preparing bicomponent fibers as described above, special equipment is necessary to channel the two separate polymer solutions into each orifice of the spinnerette so as to provide the homogeneous side-by-side arrangement of components in the fiber. Differences in the equipment useful will arise depending upon the method of spinning employed.
To produce acrylic fiber having a homogeneous side-by-side bicomponent structure and a hot water reversible spiral crimp, it has previously been necessary to employ dry-spinning procedures using suitable special equipment and organic solvents for the polymer to provide spinning compositions and subsequently consolidating the fiber by evaporation of the solvent. This procedure is effective with only a very limited number of orifices in the spinnerettes and thus is of limited productivity. Additionally, the requirement for complete solvent recovery to prevent environmental pollution further complicates production. Therefore, at the present time there continues to exist the need for a fiber possessing the desired reversible crimp and for a simplified process for preparing such a fiber. The fiber type is particularly desirable for use in craft yarn, apparel, and other end products.
In general, to produce fibers having built-in crimp, either wet or dry spinning procedures may be employed and the fibers may be eccentrically- bicomponent without the specific requirement for a side-by-side arrangement, for example, a sheath-core arrangement (although bicomponent fibers of the side-by-side arrangement are preferred for some end uses). The polymer solvent may be an organic compound or an aqueous solution of certain inorganic salts or acid.
Although a diversity of procedures may be employed to spin acrylic fibers which have irreversible helical crimp, only the organic solvent spinning procedures using relatively small spinnerettes have hereto been effective in spinning acrylic fiber having reversible spiral crimp accepted in the market place.
Since the wet-spinning procedure enables spinnerettes of very large numbers of orifices to be employed, it would be highly desirable to provide a wet-spinning procedure for producing acrylic fiber having reversible built-in crimp wherein the necessity for separately channeling the two polymer components to each orifice of the spinnerette is eliminated. Such a provision would greatly reduce the requirements for special equipment and can substantially increase productivity.
With the development of static mixing units and advances in static mixing technology [see e.g., Chem. Eng. Progress 82/7, 42-48(1986)], the utilization of such devices in fiber spinning has been studied. Many new species of multilayered bicomponent arcylic fiber types have been reported using adaptations of static mixing units in conjunction with wet-spinning procedures [see e.g., U.S. Pat. No. 4,297,412 Achard (1981); U.S. Pat. No. 4,307,054 Chion (1981); Toray European Application 330,766 Osino (1988); and Toray Japanese Applications JO-1229-812, JO-1229-813, JO-1229-814, JO-1239-127, and JO-1239-161 (all 1988)]. However, up to the present time, procedures employing static mixing units in conjunction with wet-spinning to provide acrylic fiber with a high degree of reversible crimp have not been developed.